This invention relates to novel alpha-form crystals of tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane which have good flowability and to a method of producing the same. The compound mentioned above is in wide use as an antioxidant for polyolefins and so forth.
The tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane p roducts that are on the market are in the form of fine powders and, therefore, have problems from the working environment, operation efficiency and measurability viewpoints. Thus, for example, they are readily scattered upon handling, have poor flowability and tend to adhere to containers. An improved product is desired.
Crystallographically, tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane shows the property so called polymorphism. Thus it can occur in various stable or metastable crystal forms though, chemically, they have one and the same structural formula. The crystal forms so far known include the alpha (.alpha.), beta (.beta.), gamma (.gamma.) and delta (.delta.) forms, among others.
Among these, the alpha crystal form belongs to the class of stable crystal forms. However, Japanese Patent Publication No. 13018/1985 describes the finding that while said form can be obtained with a slight loss in the step of recrystallization or, in other words, in high yields, said form cannot meet either of the requirements that it should be a powder having good properties and that it should have a high purity.
The present inventors checked the examples disclosed in Japanese Patent Publications Nos. 18617/1967 and 19083/1967 and Japanese Kokai Tokkyo Koho No. 156645/1985 by carrying out recrystallization experiments using n-heptane, n-hexane, methanol and the like. The results may be summarized as follows. The alpha crystals obtained with n-heptane or n-hexane have fairly improved flowability but have a yellow color, hence a low purity, without any marked purification efficiency. These solvents are low-boiling hydrocarbon solvents and therefore require a measure for static electricity and a special filter for solid-liquid separation, among others. Therefore, the method cannot be said to be suited for industrial application. On the other hand, methanol produces an outstanding purification effect, giving alpha crystals satisfactory from both the color and purity viewpoints. However, the product occurs as a fine powder. Therefore, none of the known methods can solve the drawbacks of the products currently on the market to give a desirable product.
As mentioned above, powder products have problems with respect to handling properties, continuous process operability and process automation, for instance. Therefore, various proposals have been made to improve the drawbacks of the currently available products. For example, a means is available which comprises avoiding the use of alpha crystals of tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane which are poor in purity and flowability and producing delta crystals, or beta crystals unstable from the crystal structure viewpoint. Japanese Patent Publications Nos. 13017/1985 and 13018/1985 propose delta crystals, which have good flowability. However, there are still problems. For instance, in the step of transesterification reaction, (1) addition is required of a particular compound, dimethyl .beta.-(3,5-di-tert-butyl-4-hydroxybenzyl)glutarate and (2) it is necessary that a molecular adduct with isopropanol should be prepared and, after solid-liquid separation and drying, be crystallized using methanol, ethanol or the like. Japanese Kokai Tokkyo Koho No. 258343/1987 proposes beta crystals having good flowability. However, the method of producing them have some problems. For example, the presence of the starting material methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and an intermediate, namely the tris substituted product, is required before crystallization, making the process complicated, causing a tendency toward decreased product purity and lowering the crystallization yield. The method cannot be said to be economical or suited for industrial application.